User Guidelines for Waste and Byproduct Materials in Pavement Construction

Approximately 280 million tires are discarded each year by American motorists, approximately one tire for every person in the United States. Around 30 million of these tires are retreaded or reused, leaving roughly 250 million scrap tires to be managed annually. About 85 percent of these scrap tires are automobile tires, the remainder being truck tires. Besides the need to manage these scrap tires, it has been estimated that there may be as many as 2 to 3 billion tires that have accumulated over the years and are contained in numerous stockpiles.(1)

Scrap tires can be managed as a whole tire, a slit tire, a shredded or chipped tire, as ground rubber, or as a crumb rubber product.

Slit tires are produced in tire cutting machines. These cutting machines can slit the tire into two halves or can separate the sidewalls from the tread of the tire.

Shredded or Chipped Tires

In most cases the production of tire shreds or tire chips involves primary and secondary shredding. A tire shredder is a machine with a series of oscillating or reciprocating cutting edges, moving back and forth in opposite directions to create a shearing motion, that effectively cuts or shreds tires as they are fed into the machine. The size of the tire shreds produced in the primary shredding process can vary from as large as 300 to 460 mm (12 to 18 in) long by 100 to 230 mm (4 to 9 in) as wide, down to as small as 100 to 150 mm (4 to 6 in) in length, depending on the manufacturer, model, and condition of the cutting edges. The shredding process results in exposure of steel belt fragments along the edges of the tire shreds.(3) Production of tire chips, which are normally sized from 76 mm (3 in) down to 13 mm (1/2 in), requires two-stage processing of the tire shreds (i.e., primary and secondary shredding) to achieve adequate size reduction. Secondary shredding results in the production of chips that are more equidimensional than the larger size shreds that are generated by the primary shredder, but exposed steel fragments will still occur along the edges of the chips.(3)

Ground Rubber

Ground rubber may be sized from particles as large as 19 mm (3/4 in) to as fine as 0.15 mm (No. 100 sieve) depending on the type of size reduction equipment and the intended application.

The production of ground rubber is achieved by granulators, hammermills, or fine grinding machines. Granulators typically produce particles that are regularly shaped and cubical with a comparatively low-surface area. The steel belt fragments are removed by a magnetic separator. Fiberglass belts or fibers are separated from the finer rubber particles, usually by an air separator. Ground rubber particles are subjected to a dual cycle of magnetic separation, then screened and recovered in various size fractions.(4)

Crumb Rubber

Crumb rubber usually consists of particles ranging in size from 4.75 mm (No. 4 sieve) to less than 0.075 mm (No. 200 sieve). Most processes that incorporate crumb rubber as an asphalt modifier use particles ranging in size from 0.6 mm to 0.15 mm (No. 30 to No. 100 sieve).

Three methods are currently used to convert scrap tires to crumb rubber. The crackermill process is the most commonly used method. The crackermill process tears apart or reduces the size of tire rubber by passing the material between rotating corrugated steel drums. This process creates an irregularly shaped torn particle with a large surface area. These particles range in size from approximately 5 mm to 0.5 mm (No. 4 to No. 40 sieve) and are commonly referred to as ground crumb rubber. The second method is the granulator process, which shears apart the rubber with revolving steel plates that pass at close tolerance, producing granulated crumb rubber particles, ranging in size from 9.5 mm (3/8 in) to 0.5 mm (No. 40 sieve). The third process is the micro-mill process, which produces a very fine ground crumb rubber in the size range from 0.5 mm (No. 40 sieve) to as small as 0.075 mm (No. 200 sieve).(4)

In some cases, cryogenic techniques are also used for size reduction. Essentially, this involves using liquid nitrogen to reduce the temperature of the rubber particles to minus 87oC (-125oF), making the particles quite brittle and easy to shatter into small particles. This technique is sometimes used before final grinding.(5)

Additional information on the production and use of scrap tire products can be obtained from:

Scrap Tire Management Council

1400 K Street, N.W.

Washington, D.C. 20005

CURRENT MANAGEMENT OPTIONS

Recycling

About 7 percent of the 250 million scrap tires generated annually are exported to foreign countries, 8 percent are recycled into new products, and roughly 40 percent are used as tire-derived fuel, either in whole or chipped form.(1)

Currently, the largest single use for scrap tires is as a fuel in power plants, cement plants, pulp and paper mill boilers, utility boilers, and other industrial boilers. At least 100 million scrap tires were used in 1994 as an alternative fuel either in whole or chipped form.(1)

At least 9 million scrap tires are processed into ground rubber annually. Ground tire rubber is used in rubber products (such as floor mats, carpet padding, and vehicle mud guards), plastic products, and as a fine aggregate addition (dry process) in asphalt friction courses. Crumb rubber has been used as an asphalt binder modifier (wet process) in hot mix asphalt pavements.(1)

As previously noted, of the roughly 30 million tires that are not discarded each year, most go to retreaders, who retread about one-third of the tires received. Automobile and truck tires that are retreaded are sold and returned to the marketplace. Currently there are roughly 1,500 retreaders operating in the United States, but the number is shrinking because there is a decline in the market for passenger car retreads. The truck tire retread business is increasing and truck tires can be retreaded three to seven times before they have to be discarded.(1)

Disposal

Approximately 45 percent of the 250 million tires generated annually are disposed of in landfills, stockpiles, or illegal dumps.

As of 1994, at least 48 states have some type of legislation related to landfilling of tires, including 9 states that ban all tires from landfills. There are 16 states in which whole tires are banned from landfills. Thirteen other states require that tires be cut in order to be accepted at landfills.(6)

MARKET SOURCES

About 80 percent of all scrap tires are handled by retail tire vendors. The remaining 20 percent are handled by auto dismantlers. These two industrial groups, while not the generators of scrap tires, collect and store tires until they are picked up by transporters, sometimes referred to as "tire jockeys". These transporters take the tires to retreaders, reclaimers, and grinders or slitters or to tire disposal sites (landfills, tire stockpiles, or illegal dumps).(1)

Figure 16-1 presents a graphical overview of the scrap tire industry.

Figure 16-1. Overview of the scrap tire industry.

Since tires are combustible, tire storage sites can be potential fire hazards. Care must be taken to safeguard against carelessness or accidental ignition, which can occur at tire storage facilities.(7)

Tire shreds or chips would normally be available from tire shredder operators. Ground rubber or crumb rubber would normally be available from scrap tire processors There are probably 100 or more tire shredders in the United States, but there are only about 15 to 20 scrap tire processors.

HIGHWAY USES AND PROCESSING REQUIREMENTS

Embankment Construction - Shredded or Chipped Tires

Shredded or chipped tires have been used as a lightweight fill material for construction of embankments. However, recent combustion problems at three locations have prompted a reevaluation of design techniques when shredded or chipped tires are used in embankment construction.(7)

Aggregate Substitute - Ground Rubber

Ground rubber has been used as a fine aggregate substitute in asphalt pavements. In this process, ground rubber particles are added into the hot mix as a fine aggregate in a gap-graded friction course type of mixture. This process, commonly referred to as the dry process, typically uses ground rubber particles ranging from approximately 6.4 mm (1/4 in) down to 0.85 mm (No. 20 sieve).(4) Asphalt mixes in which ground rubber particles are added as a portion of the fine aggregate are referred to as rubberized asphalt.

Asphalt Modifier - Crumb Rubber

Crumb rubber can be used to modify the asphalt binder (e.g., increase its viscosity) in a process in which the rubber is blended with asphalt binder (usually in the range of 18 to 25 percent rubber). This process, commonly referred to as the wet process, blends and partially reacts crumb rubber with asphalt cement at high temperatures to produce a rubberized asphalt binder. Most of the wet processes require crumb rubber particles between 0.6 mm (No. 30 sieve) and 0.15 mm (No. 100 sieve) in size. The modified binder is commonly referred to as asphalt-rubber.

Asphalt-rubber binders are used primarily in hot mix asphalt paving, but are also used in seal coat applications as a stress absorbing membrane (SAM), a stress absorbing membrane interlayer (SAMI), or as a membrane sealant without any aggregate.

Retaining Walls - Whole and Slit Tires

Although not a direct highway application, whole tires have been used to construct retaining walls. They have also been used to stabilize roadside shoulder areas and provide channel slope protection. For each application, whole tires are stacked vertically on top of each other. Adjacent tires are then clipped together horizontally and metal posts are driven vertically through the tire openings and anchored into the underlying earth as necessary to provide lateral support and prevent later displacement. Each layer of tires is filled with compacted earth backfill.(8) This type of retaining wall construction was initially performed in California.

Slit scrap tires can be used as reinforcement in embankments and tied-back anchor retaining walls. By placing tire sidewalls in interconnected strips or mats and taking advantage of the extremely high tensile strength of the sidewalls, embankments can be stabilized in accordance with the reinforced earth principles. Sidewalls are held together by means of metal clips when reinforcing embankments, or by a cross-arm anchor bar assembly when used to anchor retaining walls.(8)

MATERIAL PROPERTIES

Physical Properties

Shredded Tires

Tire shreds are basically flat, irregularly shaped tire chunks with jagged edges that may or may not contain protruding, sharp pieces of metal, which are parts of steel belts or beads. As previously noted, the size of tire shreds may range from as large as 460 mm (18 in) to as small as 25 mm (1 in), with most particles within the 100 mm (4 in) to 200 mm (8 in) range. The average loose density of tire shreds varies according to the size of the shreds, but can be expected to be between 390 kg/m3 (24 lb/ft3) to 535 kg/m3 (33 lb/ft3). The average compacted density ranges from 650 kg/m3 (40 lb/ft3) to 840 kg/m3 (52 lb/ft3).(3)

Tire Chips

Tire chips are more finely and uniformly sized than tire shreds, ranging from 76 mm (3 in) down to approximately 13 mm (1/2 in) in size. Although the size of tire chips, like tire shreds, varies with the make and condition of the processing equipment, nearly all tire chip particles can be gravel sized. The loose density of tire chips can be expected to range from 320 kg/m3 (20 lb/ft3) to 490 kg/m3 (30 lb/ft3). The compacted density of tire chips probably ranges from 570 kg/m3 (35 lb/ft3) to 730 kg/m3 (45 lb/ft3).(9) Tire chips have absorption values that range from 2.0 to 3.8 percent.(10)

Crumb rubber used in hot mix asphalt normally has 100 percent of the particles finer than 4.75 mm (No. 4 sieve). Although the majority of the particles used in the wet process are sized within the 1.2 mm (No. 16 sieve) to 0.42 mm (No. 40 sieve) range, some crumb rubber particles may be as fine as 0.075 mm (No. 200 sieve). The specific gravity of crumb rubber is approximately 1.15, and the product must be free of fabric, wire, or other contaminants.(4)

Chemical Properties

Tire chips and tire shreds are nonreactive under normal environmental conditions. The principal chemical component of tires is a blend of natural and synthetic rubber, but additional components include carbon black, sulfur, polymers, oil, paraffins, pigments, fabrics, and bead or belt materials.(2)

Mechanical Properties

Limited data are available on the shear strength of tire chips, while little or no such data are available on the shear strength of tire shreds. The wide variation in shred size makes it difficult, if not virtually impossible, to find a large enough apparatus to perform a meaningful shear test. Although the shear strength characteristics of tire chips vary according to the size and shape of the chips, internal friction angles were found to range from 19o to 26o, while cohesion values ranged from 4.3 kPa (90 lb/ft2) to 11.5 kPa (90 to 240 lb/ft2). Tire chips have a permeability coefficient ranging from 1.5 to 15 cm/sec.(10)

Other Properties

Scrap tires have a heating value ranging from 28,000 kJ/kg (12,000 Btu/lb) to 35,000 kJ/kg (15,000 Btu/lb).(2) As a result, given appropriate conditions, scrap tire combustion is possible and must be considered in any application.

Tire chips can also be expected to exhibit high insulating properties. If tire chips are used as a fill material in subgrade applications, reduced depth of frost penetration compared with that of granular soil can be expected.(11)